1. Field of the Invention
This invention relates to engine control and monitoring.
2. Prior Art
Many air/fuel control systems in present-day vehicles use a HEGO (heated exhaust gas oxygen) sensor located in front of a catalyst to provide the air/fuel feedback signal for closed-loop air/fuel operation. There are, however, two significant problems that exist with this arrangement. The first problem is that the HEGO sensor can be poisoned by certain components in the raw exhaust gas, thus causing the sensor characteristics to change with time. The second problem is that the HEGO sensor may not be able to bring the exhaust gases flowing by it to chemical equilibrium, thus causing the sensor to exhibit air/fuel offset errors which are dependent on engine load and cylinder-to-cylinder air/fuel maldistribution. (See, for example, SAE paper #800018 "Non-Ideal Properties of Zr0.sub.2 and Ti0.sub.2 Exhaust Gas Oxygen Sensors" by M. A. Shulman and D. R. Hamburg.)
These problems can be solved by placing the feedback HEGO sensor after the catalyst so that the sensor will be protected by the catalyst, and the exhaust gases will be brought to chemical equilibrium. Although this arrangement is being planned for many future vehicles, it still requires the use of a HEGO sensor in front of the catalyst to provide fast closed-loop time response. In general, the time response of the HEGO sensor located after the catalyst is very slow because of the storage characteristics of the catalyst It would be desirable to combine the output of a HEGO sensor located in front of the catalyst with that of a HEGO sensor located after the catalyst in such a way as to provide a single composite signal having the low frequency components of the post-catalyst sensor and the high frequency components of the pre-catalyst sensor.